For all the heated talk of the US military's strategic 'pivot' to the region, the scene in Guam's Apra Harbour over recent days is a reminder that the shift has been well under way for some time.

Six US submarines surfaced in Guam - the closest piece of US territory to the Chinese coast - for resupply and repair in the biggest public display of the US Navy's forward-deployed submarine presence in a decade, according to a little-noticed naval announcement.

The vessels represent the leading edge of a repositioning of US submarines from the Atlantic to the Pacific in recent years along a 60:40 split - precisely the formula for the broader US fleet announced by Defence Secretary Leon Panetta two weeks ago in Singapore.

Completing a policy hatched during the administration of president George W. Bush, some 31 of the US Navy's 53 fast attack submarines are now based in the Pacific - including three in Guam, due east of the Philippine capital, Manila.

Though far less visible than other naval assets, submarine deployments are among the most closely scrutinised given the highly sensitive nature of their work. While they are equipped with weapons such as torpedoes and guided missiles, it is their other, more discreet capabilities that attract interest - an expanding range of intelligence-gathering devices, and the ability to track rival submarines and surface ships and to release special-forces teams secretly.

'The Pentagon's forward deployment of its submarines is the harder edge of its presence in the region,' a veteran Asian military attache said. 'None of this is lost on regional militaries, and China is paying particularly close attention.

'You always have to remember that submarines routinely do things that would be either highly provocative or impossible for a surface ship to attempt in peacetime, like penetrate a coastline or closely shadow a foreign vessel or submarine. And when it comes to other nations' territorial waters, it has always been a case of 'just don't get caught'.'

Analysts and military attaches said North Korea was a natural target for US submarine activity, as was China's expanding naval and maritime-surveillance fleet.

With US nuclear-powered fast attack submarines capable of sustaining speeds beyond 20 knots indefinitely, the recently improved Guam facilities put the coasts of East Asia within easy range. The PLA Navy's submarine base at Yilin near Sanya on Hainan Island - the closest point on the mainland's coast to deep-water trenches leading out of the South China Sea - is widely believed to be one key target of interest.

China sees even routine submarine passages off its coasts as highly sensitive, highlighting a key difference of opinion over military activities in international waters.

Chinese officials warn against US military surveillance activity off the country's coasts - objecting to, for example, the entry of US aircraft- carrier battle groups to the Yellow Sea. US officials, meanwhile, insist that all militaries have the right of routine passage through international waters - and surveillance can be considered a routine military activity.

The US announcement confirmed that the Los Angeles-class submarines USS Buffalo and USS Chicago were among the ships in Apra Harbour. The Buffalo and Chicago are based in Guam, while other submarines are stationed along a chain of Pacific bases.

US officials have told regional counterparts that the capabilities involved in the so-called pivot are as important as the numbers of ships involved in the switch in priorities from the Atlantic to the Pacific, which is scheduled for completion in 2020. The US is aiming to exploit its latest technology in the push.

American officials have been at pains to offset concerns that US power is declining by repeatedly stressing long-held commitments to the region, as well as newer partnerships.

'It is not just the numbers - it is also what those platforms, what those units bring to the table,' Associated Press quoted US Pacific commander Admiral Cecil Haney as saying.